This invention relates to the field of power measurement. More specifically, this invention relates to efficient and accurate measurement of electrical power consumption.
Most household devices run on electrical power. In fact, a majority of personal and business devices in today's economy run on electrical power. Thus, accurately measuring electrical power consumption has become of paramount importance. Instruments used for measuring power are known as watthour meters. Watthour meters may be single-phase or multi-phase systems. Single-phase systems measure the power from a single incoming voltage and current. Multi-phase systems are used to measure power from a multi-wave power source. A multi-phase power source is an electrical system with multiple waveforms of power superimposed upon one another with a phase shift between waveforms. Typical electrical systems are one phase, two phases, or three phases. Note that power meters and watthour meters are used interchangeably throughout this specification to represent power measurement devices.
Typically, power companies maintain watthour meters of the old mechanical kind in every household. These meters are intended to provide a power company a measure of how much electrical power a household uses during a specified period for billing and statistical purposes. Historical knowledge of how much power all customers of a utility uses helps determine power generation requirements. Thus, a utility company with accurate measure of historical power usage of its customers is able to bill accurately and build new power infrastructures in anticipation of increasing demands. Accurate measure of power is important to a utility to avoid waste due to excess power generating capacity.
Power is measured at instants of time, however determination of total power consumption requires integration of power used over a specified period of time. Electrical power is the product of voltage (V) and current (I). Thus watthour meters generate the product of voltage and current and accumulate (i.e., integrate) the product over time.
In multi-phase systems, the power equation varies depending on the desired combination of current and voltages from the different phases. With advancement in digital technology, new watthour meters perform the power computation digitally thus the desired power equation may be programmed in a microcomputer. For digital computation of power, the voltage and current, which are typically analog, are sampled and converted to their digital representation before the product is computed. In multi-phase systems, several voltage and current products (V*I) are calculated and summed together to generate power.
Watthour meters have current transformers, or their equivalent, and voltage dividers, or their equivalent, for each phase of the electrical system to be measured. In digital systems, it is important that the voltage and current for each phase are sampled at the same time so as to accurately preserve their relative phase. Thus, prior art watthour meters have at least two Analog to Digital Converters (ADCs)—one for voltage and the other for current. Multi-phase meters typically have multiplexers at the input of the ADCs to permit simultaneous conversion of the voltage and current inputs from each phase.
Although simultaneous conversion of the V and I waveforms preserves their relative phase, it requires at least two Analog to Digital Converters. This consumes extra power and requires a relatively large die area when implemented in integrated circuits (ICs). Additionally, it means the current waveform, which might be quite small, sometimes less than one percent (1%) of the voltage amplitude, is being processed at the same time as the larger voltage waveform creating a potential for crosstalk between the voltage and current channels. Crosstalk from the voltage channel into the current channel can create unacceptable errors thus significantly degrading the accuracy of the watthour meter. Crosstalk can couple in through power supplies, voltage references, or the die substrate. It can also occur as electrostatic or inductive coupling from nearby conductors.
In actual practice, most watthour meter devices don't actually sample V and I exactly simultaneously. Instead, they permit one of the samples to be delayed slightly in order to compensate for minor phase shifts occurring elsewhere in the meter such as in voltage attenuators (or dividers) or current transformers. These phase adjustments are typically just a few microseconds—far smaller than the one hundred microseconds (100 usecs) or so between conversion samples.